Branching can have a dramatic effect on the properties of polymers, and bisphenol A polycarbonate (BPA-PC) is no exception. We describe here the results of a density functional (DF) study of branching that can occur during transesterification reactions in BPA-PC in the presence of catalysts. We find that sodium phenoxide (NaOPh) can lead to different branched products, because the Na ion can bond to different O atoms simultaneously. Tetraphenylphosphonium phenoxide, which can be used as an alternative catalyst to NaOPh in BPA-PC synthesis, does not lead to branching, because steric hindrance prevents its active sites from approaching the O atoms.